The measurement of oil and water flow rate in each producing zone of an oil well is important to the monitoring and control of fluid movement in the well and reservoir. In addition to a flow meter, each zone may have a valve to control the fluid inlet from that zone. By monitoring flow rates of oil and water from each zone and reducing flow from those zones producing the highest water cut (i.e., ratio of water flow rate to total flow rate), the water production of the entire well can be controlled. This, in addition, allows the reservoir oil to be swept more completely during the life of the well.
Typically, flow meters determine the holdup, i.e. the volume fraction of the gas, oil or water phase, and the velocity of the gas phase, the oil phase, or the water phase. The flow rate of water is then determined from the product of water holdup αw, the pipe area A, and the velocity of water Uw. An analogous relation holds for oil and gas flow rate. Other parameters of interest such as the slip ratio of velocities of the different phases, the gas-liquid ratio (GLR), the water-liquid ratio (wlr) and the gas-oil ratio (GOR) may be determined in the same multiphase measurement.
According to known methods, the holdups may be measured using a gamma-ray, dual energy fraction meter. Based on knowledge of the chemical composition of the gas, oil and water, such a meter conventionally measures the gas holdup, and one of the water or oil holdup (the requirement for the sum of all three holdups to equal one allows the other of the water or oil holdup to be calculated). That is, three compositional inputs allow the determination of two holdup outputs.
A number of methods of flow measurement are known such as capacitance, microwave, acoustic methods are know and extensively described in the relevant literature. Some of which use direct measurement or determine velocities through secondary measurements such as correlation measurements or are exploiting the Doppler effect.
Another method of velocity measurement uses a Venturi. In single phase flow, a Venturi generally obeys the Bernoulli equation which relates volumetric flow rate Q to fluid density ρ and pressure drop from the inlet to the throat of the Venturi.
A common method to determine the holdup in a flow of gas, oil and water is to measure the average density of the fluid. Since oil at downhole pressure and temperature typically has a density which is smaller than that of water (around 0.7 g/cm3 compared to 1.0 g/cm3), and the gas density even smaller, the respective holdups αo, αw and αg can be determined proportionately from the mixture density using for example the known densities of the components. For that purpose modern densitometers use a dual energy gamma-ray beam probing the absorption or scattering of high energy photons at two different energy levels, the so-called high-energy and low-energy beam.
Examples of the Venturi based multiphase flowmeters are described in the U.S. Pat. Nos. 4,856,344; 6,776,054; 6,265,713 and 6,286,367, which are hereby incorporated by reference, the latter two references relating to flow meter devices comprising a Venturi meter and a gamma-ray, dual energy fraction meter. A flow meter device comprising a Venturi meter and a gamma-ray, dual energy fraction meter is commercially available from Schlumberger (RTM).